a lithium battery tops at 4.2V (can actually go up to 4.3V, but I'd call that anything but SAFE).

So it does lower the fed voltage within the 3DS again. Dunno what controller is used for that, but I doubt it would have issues with 5V instead of the 4.6V.

I'd say, its pretty much save, but keep in mind:
USB ports only need to deliver 500mA. They may output more, but don't have to. The 3DS will pull whatever it can (I expect ~800-900mA)
Especially laptops tend to not deliver much when they aren't currently being charged. Some shitty MacBooks even disable the USB ports if you try to suck more than ~600mA!
Bottom line: Charging via USB will probably be slower.

I got this cable a while back when I got my DSTWO and I never used it. I saw that the 3ds charger port is the same and may want to charge it in the car while my girl friend drive us back to college.

Click to expand...

I have a similar usb charger wire which came with a bundle kit for the DSi. I've been using it successfully to charge a 3ds XL using a wall adapter meant for the iPhone. Charging time seems no different from what you get using the included adapter.

I have not yet tried to use it on a car lighter adapter, battery pack, or computer USB port but I'm fairly sure it will work fine. As for the charging time it will probably depend on the adapter, battery pack, and computer USB ports themselves.

A 900 mAH draw from the 3ds is fairly modest though. There are phones and tablets that draw much more than that for charging. The board of your PC will determine how much it will allow to be drawn. Good motherboards have features that will guarantee that much higher than normal amperage will be provided if the usb device needs it. Powered usb hubs will provide any shortage from its wall adapter.

As for a car lighter adapter or battery pack, just check the rated amperage it can provide. This should be printed somewhere on the device themselves.

Might as well add, that charging a battery quickly (lithium based ones at least - dunno about others) will shorten the overall life of the battery, as it is more stressful. I tend to charge lithium batteries at around 0.25A per cell whenever possible (= I have the time to wait).

I use a cheap USB to 3DS charge cable that was bought off eBay and it works fine plugged into a laptop, it just charges slower than the official charger. I haven't had the need to plug it in anywhere else yet.